A system, apparatus and method for efficient use of solar photovoltaic energy

ABSTRACT

A solar photovoltaic (PV) water heating system includes a tank including at least a first heating unit having at least first and second heating elements, at least one of which is switchable; a PV solar collector; an inverter adapted to convert the output from the PV collector to an alternating power supply; a modulator to modulate the alternating power supply from the inverter; a controller adapted to control the modulator and the switching of the or each switchable heating element; wherein the controller is adapted to control the modulator and the switchable heating elements to maximize the energy drawn from the PV collector.

BACKGROUND

This invention relates to the utilization of renewable energy. Thesources of some forms of renewable energy, such as solar and wind, havea highly variable output. For example, solar collector output varieswith the amount of insolation impinging on the collector, and thisvaries gradually with the position of the sun and rapidly with thepassing of clouds. Similarly, wind generators are subject to windfluctuations. While the invention is applicable to the different formsof renewable energy, the invention will be described primarily in thecontext of a solar photovoltaic energy system.

With the adoption of smart electricity meters, power companies cancharge different tariffs during periods of the day according to the loadon the power generator equipment.

There are many ways of using solar energy and the invention will bedescribed in the context of a solar electric storage water heater. Solarthermal water heating systems heat a heat transfer fluid and use this toheat water in a storage tank. Solar photovoltaic (PV) water heatingsystems convert solar energy to electric energy and use the electricalenergy to heat a resistive heating unit in the tank. Water heatingconsumes about a quarter of a typical household's electricity usage forhouseholds equipped with electric water heaters. Water heating alsomakes up a significant portion of gas consumption for households withgas water heating. Solar energy is tariff free.

Solar thermal water heaters have a disadvantage in relation to solarelectric water heaters in that, at low levels of insolation, if there isinsufficient solar input to heat the heat transfer fluid above thetemperature of the water in the tank, no useful energy can be collected.On the other hand, as long as there is sufficient solar input to a solarphotovoltaic (PV) collector to generate electrical output, the heatingunit can still deliver heat to the water in the tank due to the higherexergy value of the PV energy, the thermal system having no useableenergy when the temperature of the heat transfer fluid is not hotterthan the water in the tank.

There are many roof top PV systems installed in Australia. These systemswere popularised due to a generous feed in tariff that enabled theirowners to receive a reasonable payback for their investment. Some solarPV systems were designed to maximize the benefit of the feed in tariffand delivered the PV power to the utility grid system. However, homeowners with PV collectors may wish to use the PV energy to replace someor all of their utility grid power consumption. As water heating is asignificant portion of household consumption, this invention proposes asystem and method to utilize solar PV to supply a water heater.

One problem with solar energy is that it can be subject to randomfluctuations especially on partially cloudy days.

Typically, electric storage water heaters have a single heating unit.

WO2014089215 describes a method of using DC from the solar collector topower the heating unit. A disadvantage of DC is that it is difficult tointerrupt a DC current, and connector contacts can be eroded by sparkson switching the current off.

U.S. Pat. No. 5,293,447 (1994) describes a means for improvingefficiency by measuring the incoming solar energy intensity andswitching the resistance to approximate the maximum power point (MPP) atlow levels of insolation. The heating unit is driven by DC from the PVcollector. The system requires a separate heating unit for use withutility grid power.

Modulation of the current supplied to heating units is known. However,it has the problem that modulation systems may generate unacceptableamounts of electromagnetic interference or cause fluctuations in thepower system.

It is desirable to provide an efficient means of delivering energy fromthe solar photovoltaic collector to the heating unit assembly of a waterheater tank which mitigates or resolves one or more of these problems.

It is desirable to provide a means to facilitate internal consumption ofPV energy or for adapting existing systems to this end.

It is also desirable to provide an effective means of adding heat towater in a water heater tank.

It is also desirable to devise a heating assembly which can be adaptedto operate with existing water heater tanks.

SUMMARY OF THE INVENTION

According to an embodiment of the invention, there is provided a controlsystem for a variable energy source utility grid feed-in system havingat least a first energy consuming component having a first supplypriority, and a second energy consuming component having a second energysupply priority, the first energy supply priority being greater than thesecond energy supply priority, the controller receiving current flowinformation identifying current inflow from the utility grid or currentflow to the utility grid, the controller being adapted to control theflow of energy to the second energy consuming component, the controllerbeing adapted to control the deliver of energy from the variable energysource in order of priority to the first energy consuming component, thesecond energy consuming component, and the utility grid feed-in.

The control system can include a status monitoring means adapted tomonitor a condition of the second energy consuming component for use inregulating the flow of energy to the second energy consuming component.

The status monitoring means can include at least one of a thermostat ora temperature sensor.

The status monitoring means can include a battery charge monitor.

According to an embodiment of the invention, there is provided a firstcontroller for a variable energy source utility grid feed-in systemhaving a first energy consuming component having a first supplypriority, and at least a second energy consuming component having asecond energy supply priority, the first energy supply priority beinggreater than the second energy supply priority, the controller receivingstatus information from the energy storage component, the controllerreceiving current flow information identifying current inflow from theutility grid or current flow to the utility grid, the energy storagecomponent having a controllable load controlled by the controller, thecontroller being adapted to control the controllable load to prioritizedeliver energy from the variable energy source in order to the firstenergy consuming component, the second energy consuming component, andthe utility grid feed-in.

The second energy consuming component can be a water heater.

The status information can be temperature information.

The controllable load can include a modulator adapted to modulate thevariable energy source, and at least a first and a second heatingelement, wherein the modulator modulates the delivery of energy to thefirst heating element under the control of the controller.

At least the second heating element is switchable.

All heating elements can be switchable.

According to an embodiment of the invention there is provided a waterheating system having:

a water storage tank having:a temperature sensor;a heating unit having at least first and second heating elements atleast one of which is switchable;a controller adapted to receive temperature information from thetemperature sensor and current flow direction information from a currentsensor sensing the inflow or outflow of utility grid current;a modulator adapted to modulate the flow of energy from a variableenergy source to at least the first heating element under the control ofthe controller;wherein at least the second heating element is switchable under thecontrol of the controller;the controller being adapted to control the modulator and the switchingof the or each switchable heating element to prioritize delivery ofenergy from the variable energy source to internal consumption systemsbefore utility grid feed-in.

All the heating elements except the first heating element can beswitchable under the control of the controller.

Alternatively, all the heating elements can be switchable under thecontrol of the controller.

The impedance of the heating elements of a heating unit can be R, R/1,R/2 . . . R/(N−1), where R is the resistance of the first heatingelement, and N is the number of heating elements in the heating unit.

such that each successive heating element draws the same amount of poweras the power drawn by the sum of the preceding heating elements.

The variable energy source can be a photovoltaic (PV) energy source.

According to an embodiment of the invention, there is provided avariable energy supply system adapted to provide utility grid feed-in,the variable energy supply system including one or more consumptioncomponents, one of which is a water heating system, the variable energysupply system including:

a variable energy source;an energy converter to convert the output from the variable source to analternating energy supply equivalent to an alternating utility gridsupply;wherein the water heating system includes a water heater tank includingat least at least first and second heating elements, at least one ofwhich is switchable;a modulator to modulate the alternating energy supply from the inverter;a controller adapted to control the modulator and the switching of theor each switchable heating element to prioritize delivery of energy fromthe variable energy source to internal consumption systems beforeutility grid feed-in.

The system can include a bidirectional utility grid current sensoradapted to indicate to the controller the direction of energy flow to orfrom the utility grid, wherein the controller increases the energy tothe heating elements until all the energy from the variable energysource is consumed by the internal consumption systems or until energyis drawn from the utility grid.

A heating unit can have N heating elements, a first element can have anpower rating of V²/R1, a second heating element can have power rating ofV²/R1, and the remaining elements can have power ratings increasing byV²/R1 to a power rating of (N−1)*V²/R1. The heating elements can beconnected in parallel.

The or each heating unit can include first, second and third heatingelements, wherein the first element is modulated, the modulated elementhaving a first power rating of V²/R1, the second element has a secondpower rating of V²/R1, and the third element has an power rating of2*V²/R1.

Each heating element of the first heating unit can be switchable.

The variable energy source can be a solar photovoltaic (PV) energysupply system including a first temperature sensor adapted to measurethe temperature of water in the tank and to communicate the temperaturemeasurement to the controller, the controller being adapted to switchoff the delivery of energy to the tank when the temperature of the waterexceeds a threshold value.

The solar photovoltaic (PV) energy supply system can include a batterychargeable by the PV collector, the controller being adapted to switchPV energy from the PV collector to the water heater when other loads aremet and the battery is fully charged before power is diverted to thewater heater.

The solar photovoltaic (PV) energy supply system can include a utilitygrid energy connection adapted to supply energy to the first heatingunit under control of the controller when the output from the PVcollector is below a minimum value.

The solar photovoltaic (PV) energy supply system can include first andsecond multi-element heating units, and a changeover switch controllingthe neutral connection of the two heating units, corresponding elementsof the first and second heating units being controlled by the sameswitches.

According to an embodiment of the invention, there is provided a methodof utilizing solar photovoltaic energy in an impedance load, theimpedance load including two or more impedance components, at least oneof which is switchable, the method including the steps of:

converting DC energy from a solar photovoltaic (PV) collector to producean unmodulated alternating supply;modulating the unmodulated alternating supply to produce a modulatedalternating supply;applying the modulated alternating supply to one or more of the loadcomponents.

The method of utilizing solar photovoltaic energy can include the stepof:

applying the modulated alternating supply to only one of the impedancecomponents.

The method of utilizing solar photovoltaic energy can include the stepof:

monitoring the direction of energy flow to or from the utility grid.

The method of utilizing solar photovoltaic energy can include the stepof:

applying the unmodulated alternating supply to at least one otherimpedance component.

The method can include the steps of;

initially reducing the modulated alternating supply voltage to a minimumvalue, andincreasing the modulated alternating supply voltage until a maximumcurrent is drawn, oruntil the maximum modulated alternating supply voltage is reached,in the case where modulated alternating supply voltage is reached,reducing the modulated alternating supply voltage to the minimum,switching on a second impedance component,increasing the modulated alternating supply voltage applied to the firstimpedance element, andrepeating steps i) to l) until energy flow to the utility grid ceases.

The method can include the steps of:

monitoring the direction of energy flow to or from the utility grid.

The method can include the steps of:

varying the modulation of the modulated alternating supply;determining when the flow of energy to the utility grid ceases; andmaintaining the modulation at a level which maintains the energydelivered to the first impedance at or approximate to the level wherethe flow of energy to the utility grid ceases.

The first impedance component can be switchable.

All the heating elements of the first heating unit can be switchable.

According to an embodiment of the invention there is provided a methodof utilizing a variable energy source together with an alternatingutility grid supply to provide power for at least two loads, at least afirst of the loads being controllable, the utility grid supply and thevariable energy source being connected to a common conductor, whereinthe first load is prioritized after the remaining load or loads, and thevariable energy supply is adapted to deliver its available energy to theloads in priority to the utility grid supply, the method including thesteps of:

monitoring the flow of current to or from the utility grid supply;where current is flowing to the utility grid supply, increasing theenergy supplied to the first load until either:A. the flow of current to the utility grid ceases; orB. the maximum energy available from the variable energy source isdelivered to the first load.

The first load can include two or more heating elements; wherein a firstheating element is supplied from the variable energy source via acontrollable power modulator, and wherein the remaining heating elementsare switchable in a parallel configuration with the first heatingelement; and

the step of increasing the energy supplied to the first heating elementcan be performed by continually increasing the output from the powermodulator until either:C. the flow of current to the utility grid ceases; orD. the output of the modulator reaches a maximum;wherein, if the modulator output reaches the maximum,the modulator output is reduced,a second heating element is switched on in parallel with the firstheating element, andthe modulator output is continually increased, until either condition Cor condition D is reached, wherein if condition D is reached, theprocess of switching on further heating elements in parallel is carriedout until either all the heating elements are switched on and themodulator output is at the maximum; or until the flow of current to theutility grid ceases.The method can further include repeatedly reducing the modulator outputto zero, switching each of the remaining heating elements onsequentially and increasing the modulator output to its maximum or untilthe flow of current to the utility grid ceases.

According to an embodiment of the invention, there is provided a methodof operating a water heater connected to a utility grid and to avariable energy source, the heater having an upper heating unit and alower heating unit, wherein at least the upper heating unit has two ormore heating elements, the method including the steps of detecting theflow of energy from the variable energy source to the utility grid,applying a first amount of energy to the upper heating unit, andapplying a second amount of energy to the lower heating unit, increasingthe amount of energy delivered to the upper heating unit, monitoring theflow of energy from the variable energy source to the utility grid, andceasing to increase the delivery of energy from the variable energysource to the upper heating unit when the flow of energy from thevariable energy source to the utility grid ceases.

According to an embodiment of the invention, there is provided acontroller for a solar PV energy supply system adapted to provideutility grid energy feed-in, and to deliver energy to one or moreinternal consumption systems one of which is a water heating systemhaving a heating unit with one or more heating elements, wherein thesolar PV energy is converted to an alternating PV energy supplycontrollable by the controller, the controller being adapted to monitorthe direction of energy flow to or from the utility grid and to controlthe alternating PV energy supply to prioritize the energy delivered tothe internal consumption systems in preference to the utility gridfeed-in.

The water heating system includes a storage tank with at least oneheating unit and an energy modulator adapted to modulate the alternatingPV energy supply, the heating unit having at least two switchableheating elements at least one of which is supplied with energy from themodulator, the controller being adapted to control the modulator toprioritize delivery of energy to the water heater in preference to theutility grid energy feed-in.

The controller can be adapted to initially reduce the modulator outputto zero and apply the modulator output to a first heating element, andprogressively increase the modulator output to a maximum, andprogressively switch in additional heating elements as required untilmaximum energy is delivered from the PV collector or the flow of energyto the utility grid ceases.

When switching in each successive heating element, the controllerreduces the modulator output to the first heating element to zero, andthen progressively increases the modulator output to the first heatingelement.

According to an embodiment of the invention, there is provided a methodof utilizing solar photovoltaic energy in a system having a first loadcircuit, a water heater and a utility grid feed-in path, the systemincluding a controller controlling delivery of the PV energy to thewater heater, wherein the controller prioritizes the delivery of PVcollector energy to the first load, the water heater and the utilitygrid feed-in.

The method can include the step of: providing hysteresis in theswitching of elements.

Hysteresis can be provided by imposing non-zero modulated energy to theswitchable element during each switching operation.

Hysteresis can be provided by imposing delaying the switching ofelements.

The system can include a PV storage battery, and wherein the controllercan prioritize the delivery of PV collector energy to the first load,the water heater, the battery, and the utility grid feed-in.

According to an embodiment of the invention, there is provided avariable energy usage arrangement for a water heater the arrangement tocontrol energy flow from a variable energy supply and a utility gridsupply to a water heater, the arrangement including a controller, amodulator, a heating unit having at least first and second heatingelements and an attachment flange, wherein the second and any furtherheating elements being switchable, the controller being adapted tocontrol the modulator and the switchable elements, the modulator beingadapted to deliver a controllable power output to the first heatingelement under the control of the controller, the attachment flange beingadapted for sealed attachment to a water heater tank, the controllerbeing adapted to monitor the direction of current flow outwards to theutility grid supply or inwards from the utility grid supply, thecontroller being adapted to control the modulator and the switchableelements, to minimize or eliminate current flow out to the utility gridsupply.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a PV water heater arrangement according to anembodiment of the invention.

FIG. 1B illustrates an embodiment of the invention including athermostat and a power control arrangement according to an embodiment ofthe invention.

FIG. 1C illustrates an embodiment of the invention having one modulatedswitchable element and one switchable, non-modulated element.

FIG. 2A illustrates a PV water heater arrangement according to anotherembodiment of the invention.

FIG. 2B illustrates a modified circuit diagram for a dual element waterheating system according to an embodiment of the invention.

FIG. 2C shows detail of a parallel arrangement of a triac and a relay.

FIGS. 3A, 3B, 3C and 3D illustrate heating element switching plansaccording to corresponding embodiments of the invention.

FIG. 4 illustrates a flow diagram showing a method of operating a waterheating system according to an embodiment of the invention.

FIG. 5 illustrates modes of modulating AC power.

FIG. 6 illustrates a heating unit having two separately operable heatingelements.

DESCRIPTION OF THE EMBODIMENTS

The PV water heating system illustrated in FIG. 1 includes a PVcollector (1.002), an inverter (1.004), a water tank (1.020), a heatingunit (1.016) having two or more elements (1.016.1 . . . 1.016.x), atemperature sensor (1.022), a number of switches, (1.014.1A to 1.014.m),each associated with a heating element, an AC modulator (1.060), utilitygrid supply (1.050, bidirectional utility grid meter (1.052), utilitygrid switch 1.054), and controller (1.040). Because heating element1.016.1 is powered via the modulator, the switch (1.014.1A) is optional,as the modulator output can be reduced to zero.

The PV collector (1.002) is connected to inverter (1.004) which convertsthe DC voltage output from the PV collector to an alternating voltagesupply suitable for delivery to the utility grid. A water storage tank(1.020) has a first multi-element heating unit which is inserted in thelower portion of the tank via a sealed flange (1.017). While temperaturesensor (1.022) is shown inserted via flange (1.017), it could beinserter via a separate sealed opening, which could be in the top of thetank. Temperature sensor 1.022 is located to measure the temperature ofthe water proximate the heating unit with two or more individual heatingelements (1.016.1, 1016.x). It is understood that the tank may beequipped with two or more temperature sensors at different verticallocations.

Circuit (1.001) delivers power to other domestic devices. The otherdomestic uses will normally take precedence over the water heater forthe delivery of PV energy.

The output from the inverter (1.004) is connectable to at least one ofthe elements (1.016.1 . . . 1.016.x) via corresponding ones of theswitches (1.014.1A . . . 1.014.m).

In the embodiment of FIG. 1, modulator (1.060) modulates the AC inverteroutput supplied to one of the heating elements (1.016.1).

Controller (1.040) is adapted to receive system information, such assensor information from temperature sensor (1.022) via link (1.022.1),and utility grid current flow information from current sensor (1.053)via link (1.053.1). This enables the controller to monitor the directionof energy flow to or from the utility grid. The current sensor can be amodular device with internal communication capability which may enablethe current sensor to send the information to the controller by a numberof different links, such as household power line, Bluetooth, WiFi, orphysical cable. Alternatively, the current flow can be obtained from thepower utility's bi-directional meter (1.052) if the power utilityconsents to this. The current sensor provides feedback to the controlleron the effect of the adjustment of the modulator output by thecontroller.

The controller is adapted to control the switches (1.014.1 . . .1.014.m) via control links (1.014.1.1 . . . 1.014.m.1). The controlleralso controls the modulator (1.060) via link (1.060.1). The controllercan be a programmable controller or other suitable microprocessorcontrolled device adapted to respond to the inputs and to controlcircuit elements, such as switches 1.014A, 1.014B, 1.014 m. Thecontroller can control the connection of the alternating inverter outputor the utility grid power to one or more of the heating elements.

The utility grid power supply (1.050) can be connected to the heatingelements via the individual element switches and utility grid breakerswitch (1.054).

The two or more individual heating elements (1.016.1, 1016.x) can beconnected individually or in combinations of two or more elements to asource of electric power, such as solar collector 1.002 (via inverter(1.004), or utility grid power 1.050. Because element (1.016.1) ismodulated, it can be connected to two switches (1.014.1A and 1.014.1B).Controller (1.040) controls the switches such that switch (1.014.1A)connects PV supply to modulator (1.060) when PV supply is available andthe temperature of the water in the tank is below a maximum allowabletemperature (the maximum temperature threshold). When there is no PVsupply (eg, at night), the controller can open switch (1.014.1A) and thecontroller can operate switch (1.014.1B) to connect utility grid powerto heating unit if the water is below a second, normally lower,temperature threshold. The controller can also take account oftime-of-day tariffs to reduce the cost of using the utility grid power.

The resistance of the elements can be equal, or one or more of theelements can have a different resistance from the other elements. Thearrangement of FIG. 1 will be described with the heating unit havingthree heating elements (A, B, C), the highest power element (C) having aresistance of Q ohms, and the other two elements (A, B) having equalresistances of 2 Q ohms. In the example of FIG. 3, elements A and B havea power rating of 900 W, and element C has a rating of 1800 W, for a 240v AC supply, providing a combined power rating of 3600 W.

The controller receives inputs from the temperature sensor (1.022) andthe utility grid current sensor (1.053) or the bidirectional utilitygrid power meter (1.052). When a large amount of power is generated bythe PV collector, it may exceed the demand from the other domestic uses(1.001). In previous feed-in system, the excess power from the PVcollector would have been fed into the utility grid, the meter (1.052)calculating the amount of power delivered to the utility grid and thepower utility company would credit the home owner with the amount ofpower at the specified feed-in tariff.

According to an embodiment of the invention, when the controller (1.040)detects that power is flowing from the PV collector to the utility grid,it can activate the water heater circuits to divert the PV energy to thewater heater. Only if the amount of PV collector power exceeds thedemands of both the water heater and the other domestic uses is theexcess PV collector power delivered to the utility grid.

The switchable heating element configuration shown in FIG. 3 has theadvantage of enabling a continuous variation of output power from themodulator while only one element in this example, element A, which cancorrespond with element (1.016.1), is powered via the modulator (1.060).

In the exemplary embodiment, with complementary heating elements,element A is a 900 W element, element B is a 900 W element, and elementC is an 1800 W element, or more generally, elements A & B each have animpedance value of 2R, while element C has a value of R.

It is assumed that, in an initial state, the heating elements areunpowered, and the current sensor indicates current flowing out from theinverter into the utility grid. When the controller detects such astate, it initiates a process to divert the excess energy from theutility grid into the water heater, while continuously monitoring thecurrent flow direction via the current sensor. The current sensor cansample the current at a sufficiently high sampling rate to enable thecontroller to track the effect of each adjustment of the modulatoroutput.

In Stage 1, only element A is energized (switch (1.014.1A) closed). Thecontroller controls the modulator so that the output of the modulatorinitially starts at zero volts, and then increases the modulator outputuntil full power of 900 W is delivered to element A or until the currentsensor detects that current flow out to the utility grid has ceased.

If the current sensor detects that current is still flowing out to theutility grid, the controller initiates Stage 2. At Stage 2, thecontroller switches on element B at its full 900 W power while alsoreducing the modulator output to zero, so that no power is delivered toelement A. Element A can then be ramped up from zero to 900 W, giving acombined power of 1800 W from the combination of elements A and B.Again, if the current flow out to the utility grid stops before the fullpower is delivered to elements A & B, the controller will stopincreasing the output from the modulator.

In Stage 3, elements A & B are switched off, and element C is switchedon maintaining the power at 1800 W. Element A is again ramped up fromzero to 900 W, resulting in power usage of 2700 W, being the combinationof elements A and C. Again, if the current flow out to the utility gridstops before the full power is delivered to elements A & C, thecontroller will stop increasing the output from the modulator.

In Stage 4, element A is switched of and element B is switched fully onproviding an initial power consumption of 2700 W. Again, element A isswitched on and can be ramped up from zero to 900 W, resulting in 3600 Wbeing delivered to the water tank via the combination of elements A, B,and C. Again, if the current flow out to the utility grid stops beforethe full power is delivered to elements A, B & C, the controller willstop increasing the output from the modulator. If the current is stillflowing out to the utility grid, the bi-directional meter (1.052) willcontinue to credit the customer for the energy supplied.

Optionally, the utility grid can be connected to element (1.016.1) viaswitch (1.014.1B) while bypassing the modulator (1.060). When there isno useful output from the PV collector, eg, at night time, and utilitygrid power is needed to heat the water, switch (1.014.1A) is opened, sothe utility grid power is not fed via the modulator (1.060) to element(1.016.1).

The inverter can be designed to draw power from the PV collector up tothe maximum power point of the PV collector at the current level ofinsolation. When there is insufficient solar energy to fully meet theother domestic demand, the inverter ensures the delivery of theavailable PV energy to the load before utility grid power is drawn. Theinverter may do this by adjusting the phase and amplitude of theinverter output voltage relative to the utility grid voltage.

In the description of FIG. 1B, a distinction is made between thermostatand temperature sensors. A thermostat is a mechanical device whosephysical thermal characteristics are such as to change state at a settemperature. A temperature sensor can act as a thermometer to provide acontinuous reading of temperature. An additional feature of FIG. 1B isthe use of neutral switching, explained in more detail below withreference to FIG. 2B, the line A being active, and the line N beingneutral. This enables the controller [1.040] to select either the upperheating unit [1.016] or the lower heating unit [1.012].

FIG. 1B illustrates a water heater having first and second heating units[1.012] located in a lower portion of the tank, and [1.016] located inan upper portion of the tank. A thermostat arrangement including atemperature monitor [1.100] and a thermostat switch [1.102] can beprovided in a minimal configuration.

The thermostat can operate independently of the controller [1.040].

In one mode of operation of the minimal configuration without optionaltemperature sensors [1.022A], [1.022B], the thermostat can be set to anupper temperature threshold, eg, 75° C. Where there is excess PV energyas indicated by, for example, the flow of current out to the utilitygrid, and when the water in heated by the lower heating unit [1.012]reaches the upper temperature threshold as sensed by the thermostattemperature monitor [1.100], the thermostat switch [1.102] willinterrupt the flow of current to modulator [1.060] and the heatingelement switches [1.014]. This results in the excess PV energy beingdelivered to the utility grid.

Normally, power from the utility grid will only be delivered to thewater heater during off-peak periods. If utility grid power heating ofthe water in the tank is required in an off-peak period when not PVenergy is available, the controller can be programmed with the utilitytariff schedule and select the upper heating unit [1.016] so the utilitygrid power is only used to heat the water in the upper portion of thetank. If the thermostat is the only temperature sensitive device in thetank, the mains power will heat the water to the upper temperaturethreshold. This method thus limits the use of utility grid power. Theoff-peak utility grid power may be used to heat the upper portion of thetank to limit the consumption of utility grid power. A second heatingunit, such as [1.012] can be used when there is excess variable sourceenergy, such as PV energy, to heat the whole tank to the uppertemperature threshold as detected by the thermostat. Alternatively, asecond temperature sensor [1.022B] being used to monitor the temperaturein the lower section of the tank.

Optionally, at least a first temperature sensor [1.022A] can be locatedin an upper portion of the tank.

In a second configuration, including temperature sensor [1.022A], thecontroller can utilize the information from temperature sensor [1.022A]to set a second, lower temperature threshold, eg, 60° C. in the upperportion of the tank when utility grid power is being used to reduce theusage of grid power.

In a further configuration, when PV energy is used to heat the wholetank using lower heating unit [1.012], a further temperature sensor[1.022B] can be provided to measure the temperature in the lower portionof the tank. When the tank is heated to a chosen temperature threshold,the controller can switch the heating units off, and divert excess PVenergy to the utility grid.

The controller can be programmed with the off-peak times, and can alsobe adapted to receive off-peak time information via a communication linkwith the utility company, so that the controller is aware ofpre-programmed off-peak times, or so that the controller can be informedof variable load periods when it is preferable to power the water heaterfrom the utility grid when no variable source power is available.

FIG. 1C represents a system similar to that of FIG. 1A, but with areduced number of heating elements illustrates an embodiment of theinvention having one modulated switchable element 1.016.P, and oneswitchable, non-modulated element 1.016.Q. These elements can beseparately mounted in the tank, and the temperature sensor 1.022 canalso be separately mounted in the tank. This embodiment is adapted toperform the switching and modulation operations of Stage 1 and Stage 2of FIG. 3A.

FIG. 2A illustrates a PV feed-in and water heating system having twomulti-element heating units according to the invention, the systemincluding: water tank (2.020), a first heating unit (2.016) having oneor more heating elements (2.016.1 . . . 2.016.x), a second heating unit(2.012) having two or more elements (2.012.1 . . . 2.012.y), a firsttemperature sensor (2.022), a PV collector (2.002), battery chargeswitch (2.045), DC to DC regulator (2.042) which may also have anassociated smoothing filter, battery (2.044), battery output switch(2.046), inverter (2.004), a group of first element switches, (2.010.A .. . 2.010.N), each associated with an element of the first heating unit,a second group of second element switches (2.014.P to 2.014.R), an ACmodulator (2.060), utility grid supply (2.050), bidirectional utilitygrid meter (2.052), utility grid breaker switch (2.054), and controller(2.040). The modulator (2.060) is connected to supply either element(2.016.1) or element (2.012.1) depending on whether switch (2.010.A) or(2.014.P) is closed under control of controller (2.040).

In a manner similar to that discussed in relation to FIG. 1, thecontroller (2.040) receives inputs from the temperature sensor (2.024)and the bidirectional utility grid power meter (2.052).

As discussed with reference to FIG. 1, when a large amount of power isgenerated by the PV collector, it may exceed the demand from the otherdomestic uses. The controller (2.040) can deliver the PV collector powerto the following entities in order of preference:

1. Other domestic uses (2.001);2. water heater (20020);3. utility grid feed in via bidirectional meter (2.052).

In the embodiment shown in FIG. 2A, battery (2.044) can be used to storeenergy from the PV collector 2.002 when the PV collector output exceedsdemand from the premises and the water heater is at maximum temperature,in which case controller (2.040) closes battery charge over switch 2.045and enables the PV current to be directed into the battery via DCregulator (2.042) in priority before PV collector power is delivered tothe utility grid.

Battery output switch (2.046) can connect or disconnect the battery fromthe rest of the circuit. When the battery is fully charged and there isno demand from the premises, the PV power can be fed into the utilitygrid via the bidirectional meter (2.052). The battery charging systemwill normally have a charge detector to determine when the battery isfully charged.

The tank is fitted with two heating units, an upper heating unit (2.016)and a lower heating unit (2.012). Heating unit (2.016) is amulti-element heating unit (2.012.1 . . . 2.012.y) with associatedswitches (2.010.A . . . 2.010.N) and can be adapted to be connected toeither the PV supply or the utility grid supply. The lower heating unitcan have one or more elements and can be adapted to operate with utilitygrid power or PV collector power. Switch (2.054) connects the utilitygrid to the internal wiring, including the water heater and the otherdomestic uses circuit. As shown in FIG. 1, the tank can be fitted withonly one heating unit having two or more heating elements.

The controller, modulator and multi-element heating unit with attachmentflange, as shown in FIG. 6, can be provided in a form suitable forretro-fit assembly to an existing tank to replace a single elementheating unit with attachment flange. A current transformer to measurethe incoming or outgoing current would also be provided, and thetemperature sensor would be replaced or fitted with an adaptor to ensurecompatibility with the controller.

The controller can be programmed to activate the upper heating unit(2.016) to heat the upper portion of the tank before the lower heatingunit is activated.

The arrangement of FIG. 2A can also be operated in a manner to quicklydeliver heated water in the upper portion of the tank, while alsoheating the remainder of the water in the tank. A proportion of the PVenergy can be delivered to the upper heating unit [2.016], and theremainder of the excess PV energy can be delivered to the lower heatingunit [2.012]. All the heating elements of the upper heating unit can bepowered, while only one of the lower heating elements need be powered.The lower heating element will create convection circulation, so whilethe water in the upper portion of the tank heats rapidly, the water inthe remainder of the tank is also heated. The lower heating element canbe disposed asymmetrically in the tank to enhance the circulation. Thus,for example, three quarters of the available excess PV energy can bedelivered to the upper heating unit, and one quarter can be delivered tothe lower heating unit, eg, by selecting only one of the heatingelements of the lower heating unit.

FIG. 2B illustrates a wiring connection arrangement according to anembodiment of the invention, and FIG. 2C illustrates details oftriac/relay combination used in an embodiment f the invention. Theembodiment of FIG. 2B provides the same heating unit controlfunctionality as the arrangement of FIG. 2A, but requires fewerswitches. The active (A) and neutral (N) lines of the wiring circuit areshown as the arrangement of FIG. 2B utilizes neutral line switching toenable a reduction in the number of switches.

The utility grid power (2.050) and the output from the PV collector's(2.002) inverter (2.004) connect to the active and neutral lines.

All the neutral connections of each element in the first heating unitare connected together. Similarly, all the neutral connections of eachelement in the second heating unit are connected together. A thermalcutout switch [2.070] may be mandated by safety regulations.

The heating element switches 2.062, 2.017, and 2.019 include triacsconnected to controller 2.040 via links 2.062.1, 2.017.1, and 20.19.1respectively. Switches 2.017 and 2.019 are adapted to act as ON/OFFswitches and incorporate relays such as 2.017.2 with metal contacts2.017.3 in parallel with the triac 2.017.0 so the metal contacts carrythe current when the switches are closed. When the controller instructsthe switch to open, the triacs are designed to open after the relayoperates to avoid arcing of the metal contacts.

The triac 2.062 is designed to act as a modulator, so the controller canvary the amount of current passing through the triac to heating element2.016.1 or 2.013.1, depending on the state of switch 2.015. Thecontroller controls the modulator by applying a signal to the controlelectrode of the triac to turn the triac on, while removing the signalcauses the current to cease at the next zero crossing as shown in FIG.5. With a purely resistive load, the current and voltage are in phase.Using phase angle control as discussed below with reference to FIG. 5B,the triac [2.062] is switched on during each successive half cycle by ashort pulse from the controller. Thus the triac [2.062] is used as amodulator under control of the controller [2.040]. While the triac[2.062] is shown as separate from the controller [2.040] it isunderstood that the controller and the triac can be incorporated in asingle module. The controller implements a control routine based on thetemperature and current flow direction information to generate thecontrol signals for the triacs.

Triac/relay combinations (2.017) and (2.019) (discussed further withreference to FIG. 2C), control (under command of the controller) theconnection of the heating elements (2.016.2) and (2.016.3) of the firstheating unit to the active line. The connection of element (2.016.1) tothe active line is controlled by triac (2.062). Similarly, the activeconnections of elements (2.012.2) and (2.012.3) of the second heatingunit are controlled by the Triac/relay combinations (2.017) and (2.019),while the active connection of element 2.012.1 is controlled by triac(2.062). The triac (2.062) performs both the modulation and switchingfunction to deliver modulated AC power to the elements (2.016.1) and(2.012.1). Change-over switch (2.015) is adapted to complete the powercircuit to either the first heating unit or the second heating unit byclosing or opening the corresponding neutral path to the first or secondheating unit.

FIG. 2C illustrates the parallel connection of a triac (2.017.1) and arelay connection (2.017.3). The relay coil (2.017.2) operates the relayconnection. The relay connection is a metal connecting path and thus haslower conduction losses than the triac. When the combined triac/metalrelay switch is closed the current flows via the metal contact becauseof the lower resistance of the metal contact path. The triac has theadvantage of being able to implement zero-crossing switching. Thus, whenthe combined switch is to be opened, the metal relay contacts areopened, diverting the current via the triac. The triac can theninterrupt the current at the zero-crossing.

The controller is configured with the operating characteristics of themodulator, so it knows when the modulator is at its maximum outputsetting. The controller is adapted to increase the modulator output inincremental steps, and to receive current flow monitoring informationfrom the current transformer or utility grid meter so the controller canassess the result of each change in the modulator output. In addition,the inverter is adapted to set its output to correspond with the solarcollector maximum power point (MPP). FIG. 4 illustrates a method ofcontrolling the delivery of energy from the PV collector to the heatingunit:—

[step 4.102-4.104] Start condition, eg, Time (, sunrise+30 minutes) oroutput from inverter;[step 4.106] Stage 1 (Switch A) Set modulator (2.060) to zero;[step 4.108] Monitor energy flow in or out[step 4.110] If flow out, increase modulator output;[step 4.112] Check if modulator output is at maximum;[step 4.114] If not maximum, return to [step 4.108], which begins acontinuous process of monitoring the flow of current to or from theutility grid;[step 4.116] If at maximum, switch to next stage (eg, Stage 2—switchA+B) and return to [step 4.108];If [Step 4.108] indicates there is no flow out, go to [step [4.116] andcheck if there is inward flow from the utility grid;If there is no inward utility grid flow, return to step [4.108];If there is inward utility grid flow, check if modulation output is zeroat step [4.118];If modulation output is zero, return to step [4.108];If modulation output is not zero, set modulation output to zero at step[4.120] and return to [step 4.108] to continue monitoring the currentflow to or from the utility grid;

The method of FIG. 4 provides a continuous feedback process in which thecontroller monitors the flow of current to or from the utility grid, andwhen there is flow to the utility grid from the inverter, the controllerpowers the modulator until either all the available energy from theinverter is used in meeting the domestic load and partially powers thewater heater, or until both the domestic load and water heater are fullypowered from the inverter, and any surplus energy is exported to theutility grid. If there is insufficient energy available from theinverter, the inverter is designed to share the load with the utilitygrid, ensuring that all the energy from the solar collector is consumedbefore drawing power from the utility grid. The current transformer orutility grid meter provide continuous feedback to the controller as tothe effect of each adjustment of the modulator output when there issurplus PV energy being exported to the utility grid.

The modulation of the voltage in four stages can be smooth and linearfrom zero to maximum. However, other modulation schemes may beimplemented such as starting at the top of Stage 2 and then moving up ordown depending on the utility grid meter flow direction. Alternatively,where the controller monitors the actual level of energy flow as well asthe direction, this can be used by the controller to calculate astarting point modulation likely to cancel the flow, and can thenincrease or decrease modulation depending on the flow direction.

FIG. 5 shows two types AC power modulation which can be implementedusing triacs.

FIG. 5A illustrates burst fire control, in which power is modulated byswitching the current on (5.152) for a number of AC cycles and off foranother group of cycles (5.154). The burst fire control signal (BFCS)[5.151] to [5.153] from the controller [2.040] can be maintained for anumber of half cycles, and the trailing edge of the BFCS can occurbefore the end of the last half cycle as the triac will continue toconduct until the zero crossing of the last half cycle in which the BFCSwas removed. Switching can be timed to coincide with the zero crossings(5.156) of the current. By adjusting the duty cycle, the power deliveredcan be modulated. Burst fire control can cause problems such asflickering of lights when it is carried out on utility grid power.

In an alternative BFCS arrangement shown in FIG. 5A, the controller cangenerate short control pulses spanning the zero crossings at thebeginning of each half cycle for the duration of the required currentburst. The short pulses can commence before the zero crossing of theprevious half cycle and continue after the zero crossing.

FIG. 5B illustrates phase angle control, in which current is switched onfor a portion of each cycle (5.164) and is switched off for the rest ofthe cycle (5.162). The phase angle control signal (PACS) from thecontroller can be a short pulse [5.16.5], sufficient to turn the triacon, but ends within the same half cycle so that the zero crossingextinguishes the current. Again zero-crossing switching is used inswitching off to mitigate arcing. Phase angle control generates asignificant amount of electromagnetic interference (EMI) due to theasymmetric nature of the current. FIG. 5B illustrates leading edge phaseangle control suitable for use with triacs, as the zero crossing of thewaveform is used to extinguish the current.

While the embodiments of the invention may utilize phase angle controlas discussed with reference to FIG. 5B, the invention may utilize any ofthe available modes of modulation.

By limiting the amount of energy delivered by phase angle control, ie,modulating a lower power element such as a 500 W or 900 W elementinstead of modulating, for example, a single 3600 w element, the amountof interference can be limited.

FIG. 6 illustrates, by way of example, a heating unit (6.200) havingthree separately switchable heating elements (6.204, 6.205, 6.206, 6.205being largely obscured by 6.204). While only two elements are clearlyvisible to avoid an over-complex drawing, it is understood that theheating unit can have more than two or more elements. Element 6.206 hasa greater energy ratng, and hence lower resistance than elements 6.204and 6.205, it has a greater length and surface area to provide greatercontact with the water in the tank to provide more efficient heattransfer. The elements are mechanically affixed to flange 6.208, fromwhich they are electrically insulated. The elements are designed to passthrough an aperture in the tank wall. The flange is designed to sealablyclose the aperture. Each heating element has electrical terminals (showncollectively at (6.210)), which pass through the flange attachment tothe exterior of the tank. Electrical cable (6.212) and electricalconnector (6.214) enable power to be supplied to the heating elementsindividually or collectively.

The elements of a water heater with switchable elements can be switchedusing electromechanical relays. Such relays are subject to degradationover time, as physical wear and electrical erosion damage the switchcontacts. It is thus desirable to reduce the operation of theelectromechanical relays.

An embodiment of the invention proposes the use of hysteresis to reducethe number of times a relay needs to switch during the day.

In one alternative embodiment of the invention, hysteresis can beprovided by using an offset energy input for the modulated elementduring the element switching operation. The heating elements B, A, C canbe rated at 850, 1050, and 1700 watts respectively, again providing amaximum rating of 3.6 kW for the three elements in parallel. However,instead of using the lowest rated element (850 W in this embodiment) asthe modulated element, one of the higher rated elements is chosen as themodulated element. By selecting one of the higher rated elements, thefrequency with which the electromechanical relays connecting theelements to the energy supply can be reduced.

In one embodiment, such as that shown in FIG. 1A, the switching protocolfor this alternative arrangement having one modulated input element Arated at 1050 W, and two unmodulated elements, B, rated at 850 W, and C,rated at 1700 W, includes the steps of:

-   -   A. when there is excess solar energy available, switch in        element A with the modulator set to provide a first power input        level, which may be zero W;    -   B. ramp up the energy to element A until the available excess        solar input is reached or until the maximum power (1050 W) is        applied to element A;    -   C. where the available excess solar input exceeds the element        A's rated input, the energy input to the 1050 W element can be        reduced to a second value, which can be, for example, zero W        (FIG. 3B), or which may be chosen to complement element B, eg,        200 W (FIG. 3C), so that the combined rating of the modulated        element A and the unmodulated element B is equivalent to the        energy rating of element A (1050 W);    -   D. at the same time, element B, is switched in in parallel with        element A;    -   E. the energy input to element A is again ramped up until the        available excess solar input is reached or until the maximum        power (1050 W) is applied to element A, giving a combined input        of 1900 W;    -   F. where the available excess solar input exceeds the combined        rating of elements A & B, the energy to element A is again        reduced;    -   G. element B is switched off;    -   H. element C is switched in parallel with element A;    -   I. element A is ramped up to its maximum rating or until the        available excess solar input is reached;    -   J. where the energy to element A again reaches energy rating of        element A, power to element A is again reduced, element B is        switched in parallel with elements A & C; and    -   K. element A is again ramped up to its maximum rating or until        the available excess solar input is reached.

FIG. 3B shows the power delivery profile when element A is set to zeromodulation at a transition. This produces a saw-tooth profile due to thedifferences in the impedances of the elements A, B, and C not beingchosen to product a smooth profile as shown in FIG. 3A.

However, by using the controller to apply a complementary non-zeromodulation to element A at each transition, it is possible to provide asmooth linear profile with hysteresis to prevent hunting at thetransitions. As shown in FIG. 3C, where element A is switched to acomplementary, non-zero value at each transition to achieve anapproximately continuous linear range of energy input to the waterheater, there is an overlap at each transition. A first overlap between850 W and 1050 W occurs between the single element (element A)configuration and the A+B configuration. Similarly, an overlap occursbetween 1700 W and 1900 W at the A+B to A+C transition, and a thirdoverlap occurs between the A+C and A+B+C transition from 2350 W and 2750W.

These overlaps can be used as hysteresis in the switching protocolimplemented by the controller, so that a switching of theelectromechanical relays does not need to occur within these overlaps.Switching in either direction need only occur at the edges of theoverlaps. Thus, with falling solar input, switching would be programmedto occur at the lower edge of the overlap, while, for increasing solarinput, switching could be programmed to occur at the upper edge of theoverlap. This can reduce the frequency with which the electromechanicalrelays need to switch. The offset modulation of element A can be used toprovide a smooth power profile with unmatched elements, to provideswitching transition hysteresis, or both.

It is not necessary that the offset modulation cancels the saw-toothprofile of FIG. 3B entirely. Another offset modulation value can bechosen to provide sufficient hysteresis to reduce hunting duringtemporary fluctuations in solar input. FIG. 3D illustrates anarrangement in which the offset modulation applied to element A onswitching between different combinations of elements is less than thatrequired to completely eliminate the sawtooth profile, resulting in areduced sawtooth profile (heavy line X). When solar input is increasing,the switching follows the reduced sawtooth profile X as indicated bydashed line Y (offset from line X for illustrative purposes). When thesolar input is decreasing, the switching pattern follows the dotted lineZ (also offset from line X for illustrative purposes).

Example 1

Initially, the system starts with the modulation of A set to zero, and Band C switched off. In Stage 1, as the solar input increases to provideexcess soar energy, the modulation of A is increased. When themodulation of A reaches its maximum energy input (A=1050 W), themodulation of A is switched to H1 and B is switched in (Stage 2).Because B+H1<A, the modulation of A is increased so A=B+H1, and themodulation of A continues to increase as the solar input increases.Assuming the solar input begins to fall during Stage 2, switching backto Stage 1 occurs when the input equals the energy rating of B (850 W).Thus, with increasing solar input, switching from Stage 1 to Stage 2occurs at 850+H1 W, while, with falling solar input, switching fromStage 2 to Stage 1 occurs at 850 W. Similar offset procedures arefollowed between Stage 2 and Stage 3, and between Stage 3 and Stage 4.Thus the offset of modulation of A by setting its switching value to H1instead of zero provides hysteresis which prevents “hunting” of thesystem due to temporary fluctuations less than H1.

Alternative or additional methods of providing hysteresis can be used.For example, a time delay for switching the elements can be programmedinto the controller to take account of transient fluctuations of solarinput. A suitable duration of the hysteresis time delay may bedetermined empirically from meteorological observations. The time periodmay be variable, depending on the prevailing cloud coverage. In someinstances, a delay of 30 seconds may be chosen, or a longer period maybe chosen. A manual input may be provided with the controller so a usercan set the hysteresis delay, or online information may be used toselect the delay duration. The controller may be connected to, andprogrammable via a communication device providing internet access toonline cloud-cover information and local geographical locationinformation which can be used to select a suitable hysteresis timedelay.

A potential source of unwanted operation of the elctromechanical relaysis the random variation of solar input, due, for example, partial orcomplete occlusion of the solar collector, for example, when cloudsovershadow the solar collector. This may be overcome by allowing theutility grid power to deliver power to the heating elements during suchtransient events. This method of operation can also reduce the switchingof electromechanical relays. This can be achieved because the solarenergy voltage can fall below the level of the mains voltage for theperiod of the transient occlusion.

Application

Solar electric heating has an advantage over direct solar thermalheating of water because, when the insolation is insufficient to heatthe water or heat transfer fluid in the solar thermal collector to atemperature above the temperature of the water in the tank, no heat isadded to the water in the tank. Solar photovoltaic, on the other handhas the advantage that, as long as there is sufficient insolation topower the solar PV collector, energy can be added to the water in thetank. Thus solar photovoltaic heating can operate to heat the water atlower levels of insolation.

The method of combining switching and modulation provides a means forcontinuously varying the current supplied to the heating unit. Thecurrent drawn from the PV collector can be continuously varied. Thismeans that the current drawn from the PV collector can be matched to themaximum power point of the PV collector, enabling efficient use of theinsolation at all levels.

The inventive concept can be applied to solar PV water heating systemshaving one or more multi-element heating units that are controlled bycombining both modulation (varying power) and switching to achievelinear variable power control over the range zero to X kw's.

A three-element design can be chosen for a total 2 kW rating in 500 Wsteps. Changing the number elements and the modulator size (modulationincrement) allows for many different variations on the design. Theconcept can be applied to discrete elements and that the modulator mayor may not use the full rating of the individual elements in all casesto achieve the linear ramp up from 0 to the desired X kW.

An example of a tri-element heating unit can cover the range zero to 2.0Kw (@ 240 v=28.8Ω; (r=V²/P). A element indexing step of 500 W can beused as this is common to many “off the shelf”, Australian approveddevices that use Triac based power modulation control. However otherelement ratings can be used.

The combination of progressively increasing the modulator output andprogressively switching in additional elements facilitates the abilityto provide a continuous range of input power to the heating unit fromthe PV collector.

The heating elements of the present invention can be designed as areplacement for a single element, the shape and size of themulti-element heating unit being adapted as a direct replacement for anexisting single element heating unit. Thus a heating assembly withcontroller, modulator, element switching and multi-element heater can beprovided as a replacement heating system for an existing single elementwater heater.

1. A control system for a variable energy source utility grid feed-insystem having at least a first energy consuming component having a firstsupply priority, and a second energy consuming component having a secondenergy supply priority, the first energy supply priority being greaterthan the second energy supply priority, the controller receiving currentflow information identifying current inflow from a utility grid orcurrent flow to the utility grid, the controller being adapted tocontrol the flow of energy to the second energy consuming component, thecontroller being adapted to control the delivery of energy from thevariable energy source in order of priority to the first energyconsuming component, the second energy consuming component, and theutility grid feed-in.
 2. The control system as claimed in claim 1,wherein the second energy consuming component includes at least onewater heater heating element.
 3. The control system as claimed in claim1, including status monitoring means adapted to monitor a condition ofthe second energy consuming component for use in regulating the flow ofenergy to the second energy consuming component wherein the condition istemperature.
 4. The control system as claimed in claim 2, wherein the atleast one water heater heating element includes at least first andsecond heating elements, the controller controlling a modulator adaptedto modulate the energy from the variable energy source, wherein themodulator modulates the delivery of energy to the first heating elementunder the control of the controller.
 5. The control system as claimed inclaim 4, wherein at least the second heating element is switchable. 6.The control system as claimed in claim 4, wherein all heating elementsare switchable.
 7. (canceled)
 8. The control system as claimed in claim4, wherein the first and second heating elements are switchable underthe control of the control system.
 9. (canceled)
 10. The control systemas claimed in claim 1, wherein the variable energy source is aphotovoltaic (PV) energy source.
 11. (canceled)
 12. The control systemas claimed in claim 1, including a bidirectional utility grid currentsensor adapted to indicate to the control system the direction of energyflow to or from the utility grid, wherein the control system increasesthe energy to the first energy consuming component and the second energyconsuming component until all the energy from the variable energy sourceis consumed or until energy is drawn from the utility grid. 13.(canceled)
 14. The control system as claimed in claim 1, wherein thevariable energy source is a solar photovoltaic (PV) energy supply systemincluding a first temperature sensor adapted to measure a temperature ofwater in a water heater tank and to communicate the temperaturemeasurement to the control system, the control system being adapted toswitch off the delivery of energy to the water heater tank when thetemperature of the water exceeds a threshold value.
 15. The controlsystem as claimed in claim 14, wherein the solar photovoltaic (PV)energy supply system includes a battery chargeable by the PV collector,the control system being adapted to divert PV energy from the PVcollector to either the battery or the water heater.
 16. The controlsystem as claimed in claim 14, wherein the solar photovoltaic (PV)energy supply system includes first and second multi-element heatingunits, and a changeover switch controlling the neutral connection of thetwo heating units, corresponding elements of the first and secondheating units being controlled by the same switches.
 17. A method ofutilizing a variable energy source together with an alternating utilitygrid supply to provide power for at least two loads, at least a first ofthe loads being controllable, the utility grid supply and the variableenergy source being connected to a common conductor, wherein the firstload is prioritized after the remaining load or loads, and the variableenergy supply is adapted to deliver its available energy to the loads inpriority to the utility grid supply, the method including the steps of:monitoring the flow of current to or from the utility grid supply; whencurrent is flowing from the variable energy source to the utility gridsupply, increasing the energy supplied to the first load until either:A. the flow of current to the utility grid ceases; or B. the maximumenergy available from the variable energy source is delivered to thefirst load.
 18. The method as claimed in claim 17, wherein the firstload includes two or more heating elements; wherein a first heatingelement is supplied from the variable energy source via a controllablepower modulator, wherein the remaining heating elements are switchablein a parallel configuration with the first heating element; wherein thestep of increasing the energy supplied to the first heating element isperformed by continually increasing the output from the controllablepower modulator until either: C. the flow of current to the utility gridceases; or D. the output of the modulator reaches a maximum; wherein, ifthe modulator output reaches the maximum, the modulator output isreduced, a second heating element is switched on in parallel with thefirst heating element, and the modulator output is continuallyincreased, until either condition C or condition D is reached, whereinif condition D is reached, the process of switching on further heatingelements in parallel is carried out until either all the heatingelements are switched on and the modulator output is at the maximum; oruntil the flow of current to the utility grid ceases.
 19. A method ofutilizing solar photovoltaic energy in an impedance load, the impedanceload including two or more energy consuming components, at least one ofwhich is switchable, the method including the steps of: converting DCenergy from a solar photovoltaic (PV) collector to produce anunmodulated alternating supply; modulating the unmodulated alternatingsupply to produce a modulated alternating supply; and applying themodulated alternating supply to one or more of the power consumingcomponents.
 20. The method of utilizing solar photovoltaic energy asclaimed in claim 19, including the steps of: monitoring the direction ofenergy flow to or from the utility grid, and, in the case that there iscurrent flow to the utility grid, increasing the modulated alternatingsupply until a predetermined maximum current is drawn, or the currentoutflow ceases, in the case where maximum modulated alternating supplyoutput is reached, reducing the modulated alternating supply output tothe minimum, switching on a second parallel power consuming element,increasing the modulated alternating supply voltage applied to the firstpower consuming element, and reducing the modulator output, switchingthe power consuming components, and increasing the modulator output,until either the energy flow to the utility grid ceases or until all thepower consuming elements are switched on.
 21. The method of utilizingsolar photovoltaic energy as claimed in claim 20, including the step of:providing hysteresis in the switching of elements.
 22. The method ofutilizing solar photovoltaic energy as claimed in claim 21, whereinhysteresis is provided by imposing non-zero modulated energy to theswitchable element during each switching operation.
 23. The method ofutilizing solar photovoltaic energy as claimed in claim 21, whereinhysteresis is provided by imposing delaying the switching of elements.24. (canceled)
 25. (canceled)